Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions. MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
Electromagnetic tools generally use either magnetic dipole antennas, in which case they are based on induction physics, or they use electrodes (electric dipole antennas) to inject current into the formation. Typically, particularly for electromagnetic measurements, a signal originates from a tool disposed in the interior of an uncased wellbore, passes through the formation outside the wellbore, and returns to a receiver within the wellbore. Because the signal travels through the formation, certain properties of the formation can be inferred from the measurement. Measurements are typically performed in an uncased portion of the wellbore because conventional conductive casing tends to limit the electromagnetic signal that can pass between the interior and exterior of a cased wellbore.
Dielectric measurements are electromagnetic measurements used to estimate petrophysical properties such as (but not limited to) water saturation, water salinity, and hydrocarbon residual saturation, as well as textural information regarding the geometry of a porous medium. Dielectric measurement tools generally use higher frequency signals than conventional electrical resistivity tools. Several petrophysical models have been developed to interpret and link the dielectric measurements to the petrophysical properties mentioned above.
Numerous dielectric forward models exist to convert the dielectric measurements into water saturation and the other reservoir properties listed above. Such models, referred to herein as “background models”, include, but are not limited to, the Bimodal Model, the Stroud-Milton-De Model, the Shaly Sand Model, and the Complex Refractive Index Model (CRIM). Each type of model has inherent strengths and weaknesses based on the assumptions intrinsic to the model. Some model types (e.g., effective medium and phenomenological) work well with relatively different rock types, taking into account the order and shape of replacement material. These models are derived to capture different polarization effects regardless of the geometrical features of the system. Other model types (e.g., empirical and semi-empirical) can accurately predict values for the data used to construct them, but are not widely applicable to capture all the polarization effects of the dielectric dispersion. None of those models, however, adequately provides wettability information. Furthermore, conventional wettability measurements within laboratories do not fully or adequately represent the actual downhole wettability, which is often difficult to measure.